A photoreceptor for electrophotography consists of an electrically conductive support and, in contact with it, one or more layers which are insulating in the dark but become conductive upon exposure to image-forming radiation. In a typical process for forming an image, the surface of the photoreceptor is first electrostatically charged in the dark to a uniform initial surface potential and then exposed to image-forming radiation. In areas where the photoreceptor is irradiated, mobile charge carriers are generated and migrate so as to discharge the initial potential, whereas substantially no discharge occurs in unirradiated areas. The resulting imagewise pattern of surface potential can be developed into a visible image by application of a liquid or dry developer containing small, charged, black or colored toner particles. The image formed by the toner, can, if desired, be transferred to another surface, e.g., to paper.
A single-active-layer photoreceptor is one in which the generation of mobile charge by imaging light and its subsequent migration to effect a discharge occur in a single layer (the photoconductive layer). An example is Kodak Ektavolt Recording Film SO-102. An advantage of such a photoreceptor is the simplicity and economy of its fabrication. Usually, however, such a photoreceptor has a rather low sensitivity to image forming radiation.
A two-active-layer photoreceptor is one in which the generation of mobile charge by imaging light and its subsequent migration to effect a discharge occur in two distinct layers. One layer is responsible for the generation of mobile charge and the other provides a medium in which that charge can migrate. The former, termed the charge-generation layer (CGL), usually is relatively thin. The latter, termed the charge-transport layer (CTL), usually is relatively thick. Usually the CGL is sandwiched between the conductive support and the CTL. Other cases concerning the relative thicknesses and the relative positions are possible. For example, the CTL may be sandwiched between the conductive support and the CGL. In any case, the CGL and the CTL must be in intimate electrical contact. Two-active-layer photoreceptors frequently have advantageously high sensitivity to imaging radiation, smooth surfaces which can readily be cleansed of excess toner, good resistance to mechanical wear, and the ability to be used repeatedly to form a multiplicity of images. Typically they have the disadvantage of being relatively expensive to fabricate.
Photoreceptors can also be constructed with more than two electrically active layers. For example, U.S. Pat. No. 3,953,207 discloses a photoreceptor comprising, in order, a first CTL, a CGL, and a second CTL. A photoreceptor with two or more electrically active layers is termed a multi-active-layer photoreceptor.
In a single-active-layer photoreceptor, the photoconductive layer frequently comprises a monomeric charge-transporting compound, a polymeric binder and, optionally a monomeric sensitizing compound. The charge-transporting compound may be an aromatic amine or other nitrogen-containing organic compound capable of transporting positive charge carriers (holes), such as described in Columns 3-7 of U.S. Pat. No. 4,442,193 and Columns 4-5 of U.S. Pat. No. 4,666,802. Such hole-transporting compounds are often described as being "p-type photoconductors". The polymeric binder is typically of high molecular weight and otherwise chosen for (inter alia) high electrical resistivity and dielectric strength, good film-forming properties, solubility in organic solvents, and ability to act as a solid-state solvent for the monomeric components. Examples of polymeric binders are listed in Column 7 of U.S. Pat. No. 4,442,193. Various sensitizing compounds are known such as the naphthalene bis-dicarboximide compounds disclosed in U.S. Pat. No. 4,442,193 and the substituted pyrylium salts disclosed in U.S. Pat. Nos. 4,167,412 and 4,341,894. Instead of a monomerically dissolved sensitizing dye it is possible to use a finely divided inorganic, organometallic, or organic pigment dispersed through the photoconductive layer. In either case, the primary function of the sensitizing compound or the pigment is to provide for the generation of mobile charge in response to image-forming radiation.
A CTL in a two-active-layer photoreceptor often has a composition similar to that of the photoconductive layer of a single-active-layer photoreceptor, except that a CTL typically contains no sensitizing compound or dispersed pigment.
The CGL may contain an inorganic, organometallic, or organic pigment, either dispersed in a polymeric binder or present as a neat (i.e., monocomponent) layer. For examples, see U.S. Pat. No. 4,559,287, Column 6 and U.S. Pat. No. 4,346,158, Column 10. Alternatively it may contain an aggregate photoconductive composition as described in U.S. Pat. No. 4,175,960.
The concentration of a monomeric charge-transporting compound in a single-active-layer photoreceptor or in a CTL of a multi-active-layer photoreceptor can vary widely, from approximately 10 percent to about 60 percent of the dry weight of the layer. Generally, it is expected that the lower concentrations give better resistance of the photoreceptor against abrasion and a lower rate at which the photoreceptor, once charged, discharges in the dark (i.e., a lower "dark decay rate"). In addition, when the charge-transporting compound is relatively expensive, it is advantageous to use as little as is possible consistent with the requirements of a particular application. On the other hand, the lower concentrations also result in greatly reduced values of the mobility of charge carriers in the layer. Thus, a lower limit to the concentration is inevitably set at that point where the mobility is barely sufficient to accomplish a desired degree of discharge in a desired period of time. The mobility is defined as the drift velocity of charge carriers divided by the strength of the electric field. It typically depends on electric field strength, decreasing at lower field strengths. Roughly speaking, a mobility value around 1.times. 10.sup.-7 cm.sup.2 /V sec at a field strength of 5.times.10.sup.4 V/cm is adequate for most electrophotographic processes, although certain process cycles may require greater values or tolerate lesser values.
On theoretical grounds, it has been stated that "Good electron- (hole) transport is achieved by utilizing . . . either rigid or nonpolar polymer matrices" [C. B. Duke and R. J. Meyer, Physical Review B, vol. 73, pp. 2111-2125 (1981)]. Standard polymeric binders (matrices) such as bisphenol A polycarbonate, however, are fairly rigid and only moderately polar, so it is not evident that binders that are yet more rigid or less polar should result in greatly improved mobility values. Moreover, published experimental data have not shown a significant dependence of the mobility on the binder. [See W. D. Gill, Proceedings of the Fifth International Conference on Amorphous and Liquid Semiconductors, edited by J. Stuke and W. Brenig (Wiley, New York, 1974, pp 901-907) and D. M. Pai et al., Philosophical Magazine B, vol. 48, pp. 505-522 (1983)].
The time required for a particular degree of discharge to occur is affected, not only by the mobility of the charge carriers but also by how rapidly they are injected from the CGL into the CTL. The term "delayed injection" refers to the condition in which the injection process requires a time comparable to, or greater than, the time required for charge carriers to cross the CTL. The presence and/or severity of delayed injection depends on the compositions of both the CGL and the CTL in a manner that is not understood in any detail.
In addition to photoreceptors for electrophotography, other devices that employ organic materials to transport holes (and the same or different organic materials to transport electrons) include photovoltaic cells (see U.S. Pat. No. 4,164,431) and electroluminescent devices (see U.S. Pat. Nos. 4,356,429 and 4,539,507). In each such device, for a given choice of charge-transporting compound, it is desirable to increase the mobility of the holes.